Feeding ecology of dunlins Calidris alpina staging in the southern Baltic Sea, 1. Habitat use and food selection

The feeding habits of migrating dunlins Calidris alpina staging in differen t non-tidal coastal habitats in the southern Baltic Sea are described. The study also focuses on the structure of the benthic macrofauna of these habi tats and the diet choice of dunlins. All investigations were carried out on Langenwerder Island (Wismar Bay), where different types of flats and beach es harbour a total of 30 to 40 species of marine macrofauna. The compositio n of the macrobenthos differed considerably between the eulittoral sandbank , the eulittoral mudflat, the pebble beach, and the sublittoral surrounding s. Most dunlins were observed foraging in flocks of up to several hundred i ndividuals on the eulittoral flats. Densities of up to 20 to 30 foraging du nlins ha(-1) occurred annually during peak migration in September and Octob er. Macrobenthos biomass in these habitats fluctuated between 20 and 40 g A FDM m(-2). The mean total food consumption of dunlins during autumn migrati on was estimated at 0.01 g AFDM m(-2) d(-1). The predation pressure could b e estimated at 3 to 6% of the suitable food supply. Dunlins staging on Lang enwerder were able to attain a pre-migratory mass gain of 0.2 to 0.5% of th eir body weight per day within an 8 to 12-h daily feeding period. The birds fed predominantly on the polychaete Hediste diversicolor by probing. They selected small 7 to 31-mm-long individuals. When water levels were high, an d the eulittoral flats inundated, many dunlins switched to foraging along t he shorelines where a variety of small prey were taken from spilled macroph ytes. Dunlins sometimes obviously ignored their most important food H, dive rsicolor, although available, by feeding on other prey such as juvenile fis hes and shrimps, dipteran larvae or spilled amphipods. When feeding on amph ipods, dunlins selected the smallest individuals. (C) 1999 Elsevier Science B.V. All rights reserved.